1. Field of the Invention
The present invention relates to a control system for an internal combustion engine. More specifically, the invention relates to a control system for an internal combustion engine, for supplying ineffective fuel which does not burn in the combustion chamber.
2. Description of the Related Art
There has been known a technology for adjusting the air-fuel ratio of the exhaust gas from an engine independently of the engine operating air-fuel ratio (combustion air-fuel ratio in the combustion chamber) by supplying, to the engine, ineffective fuel that does not contribute to the combustion in the combustion chamber. For example, a NO.sub.x occluding and reducing catalyst is disposed in the exhaust passage of the engine which operates at a lean air-fuel ratio, the NO.sub.x occluding and reducing catalyst absorbing NO.sub.x in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean, and releasing and purifying by reduction the absorbed NO.sub.x when the air-fuel ratio in the exhaust gas becomes rich. In this case, the air-fuel ratio of the exhaust gas flowing into the NO.sub.x occluding and reducing catalyst must be set to be rich at regular intervals and the NO.sub.x must be released from the NO.sub.x occluding and reducing catalyst, so that the NO.sub.x occluding and reducing catalyst will not be saturated with NO.sub.x when the engine is operated at a lean air-fuel ratio. In such a case, a change in the engine operating air-fuel ratio from the lean side to the rich side increases the engine output torque; i.e., a change in the air-fuel ratio changes the torque. Upon supplying ineffective fuel that does not contribute to the combustion in the engine combustion chamber, i.e., that does not burn in the engine combustion chamber, therefore, it becomes possible to advantageously change only the air-fuel ratio of the exhaust gas independently from the engine operating air-fuel ratio. In an engine having direct cylinder fuel injection valves for directly injecting the fuel into the cylinders, the ineffective fuel can be supplied into the cylinders by secondary fuel injection in the expansion stroke or in the exhaust stroke of the cylinders. In an engine having exhaust port fuel injection valves for injecting the fuel into the exhaust port of the engine, further, the ineffective fuel can be supplied into the exhaust ports by the exhaust port fuel injection.
The fuel injected into the cylinder during the expansion stroke or the exhaust stroke or the fuel injected into the exhaust port of the cylinder is vaporized without being burned and is discharged together with the exhaust gas. That is, the ineffective fuel that is supplied does not contribute to the combustion in the engine, but the amount of the unburned HC component in the exhaust gas from the engine increases by an amount of the ineffective fuel that is supplied to establish a rich air-fuel ratio. By supplying the ineffective fuel to the engine, therefore, it is possible to change the air-fuel ratio only in the exhaust gas from the engine without affecting the engine operating air-fuel ratio.
A device for supplying the ineffective fuel of this type has been disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-212961.
According to the device of this publication, a NO.sub.x occluding and reducing catalyst is disposed in the exhaust passage of a diesel engine to absorb the NO.sub.x in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean and to release the NO.sub.x when the oxygen concentration has decreased in the exhaust gas that is flowing in. Under normal condition, the main fuel is injected into the cylinder near the compressive top dead center of the cylinder of the engine and, when the NO.sub.x is to be released from the NO.sub.x occluding and reducing catalyst, the secondary fuel is injected during the expansion stroke or the exhaust stroke of the engine in addition to injecting the main fuel. The fuel injected into the cylinder during the expansion or exhaust stroke does not contribute to the combustion in the cylinder, i.e., does not burn in the cylinder and is exposed to the burned gas of a high temperature in the cylinder. Therefore, hydrocarbons having large molecular weights in the fuel are decomposed into hydrocarbons having small molecular weights. Besides, the fuel supplied by the secondary fuel injection does not contribute to the combustion but is simply discharged from cylinders together with the exhaust gas. By supplying the ineffective fuel to the engine, therefore, it is made possible to inject the fuel in a relatively large amount for establishing a rich air-fuel ratio in the exhaust gas without increasing the pressure of an explosion in the cylinder even in a diesel engine. According to the device of the above-mentioned publication, when the secondary fuel is injected, the exhaust gas having a rich air-fuel ratio, containing a large amount of hydrocarbons of low molecular weights which are highly active, flows into the NO.sub.x occluding and reducing catalyst in the exhaust passage. When the secondary fuel is injected, therefore, the NO.sub.x that has been absorbed is released from the NO.sub.x occluding and reducing catalyst and is purified by reduction with hydrocarbons in the exhaust gas.
In an engine which effects the secondary fuel injection as done by the device of the above-mentioned publication, however, the fuel supplied by the secondary fuel injection is not completely exhausted during the exhaust stroke but often remains in the cylinder. When the fuel of the secondary fuel injection partly remains in the cylinder, this remaining fuel burns in the cylinder in addition to the fuel supplied by the main fuel injection at the time when the main fuel is injected next time. Accordingly, an amount of fuel burnt in the engine increases, whereby an increased torque is produced by the combustion. This causes a change in the output torque of the engine.
When the ineffective fuel is supplied to the engine by the exhaust port fuel injection without relying upon the secondary fuel injection, on the other hand, the fuel does not remain in the cylinder. In an engine equipped with an exhaust gas recirculation (EGR) device, however, the similar problem may occur when the exhaust port fuel injection is effected.
There has been generally known an exhaust gas recirculation (EGR) device in which the exhaust gas from the engine is partly recirculated into the combustion chamber of an internal combustion engine to lower the combustion temperature in the combustion chamber in order to decrease the amount of NO.sub.x (nitrogen oxides) formed by the combustion. The exhaust gas recirculation system includes an external EGR system in which an exhaust passage of the engine is connected to an intake passage of the engine through an. EGR passage, and the amount of the exhaust gas to be recirculated is adjusted by a flow rate adjusting valve (EGR valve) provided in the EGR passage, and an internal EGR system by which the amount of blow back of the burned gas in the combustion chamber caused by the overlapping of valve is adjusted by changing the open-close timings of the intake valve and the exhaust valve of the engine.
When the ineffective fuel is supplied to the engine that utilizes the EGR (exhaust gas recirculation) as described above, there a problem occurs not only when the ineffective fuel is supplied by the secondary fuel injection but also when the ineffective fuel is supplied by the exhaust port injection.
That is, when the ineffective fuel is supplied as described above, the exhaust gas from the engine contains unburned fuel in relatively large amounts. When the exhaust gas is directly recirculated by the EGR device into the combustion chamber of the engine, part of the ineffective fuel that should not burn in the combustion chamber is recirculated into the combustion chamber and burns therein. When the ineffective fuel is supplied while the EGR is being performed, therefore, the fuel is supplied in an excess amount into the engine and the combustion air-fuel ratio becomes excessively rich, whereby the combustion in the combustion chamber becomes unstable or the output torque of the engine increases due to the combustion of excess fuel.